Mobile devices are widely used in today's world. The arrival of multi-functional mobile devices (e.g., smartphone or tablet computer) makes it much more convenient for people to be in contact with each other. As a result, many parents now give mobile devices to their children and use the mobile devices to track down their children's activities when they are not with their parents. But today's approaches are too intrusive to the children's privacy or too inconvenient (e.g., a child is asked to make a phone call or send a text message after arriving at certain locations) or both.